The present invention relates to blow molding of thermoplastic container bodies. More particularly, this invention relates to the production of three-piece containers formed from tandem container bodies, blow molded from a hollow, thermoplastic preform having a neck finish on both ends thereof.
Blow molding of thermoplastics has been taught in the prior art. This art generally teaches the process as involving the steps of forming a hollow tube, heating the tube, then introducing a gas under pressure into the hollow tube. The pressurized gas expands the heated tube against a mold to form a shaped article. When the hollow tube is stretched mechanically in the longitudinal direction either before or during expansion with pressurized gas, the process is sometimes called stretch-blow molding.
Stretching of a thermoplastic material both longitudinally and transversely results in a high degree of biaxial orientation of the molecular structure of the thermoplastic material. Such biaxial orientation provides favorable properties including high tensile strength and yield strength, toughness or high resistance to impact, low creep at low weight to volume ratios and sufficient barrier properties which render this material useful in containing liquids under pressure.
Blow molding and stretch-blow molding are taught in the prior art including U.S. Pat. No. 4,447,199. The simultaneous production of multiple containers by blow molding a single hollow tube has also been taught in the prior art, including U.S. Pat. Nos. 3,457,590, 4,103,411 and 4,796,766.
One of the many challenges encountered in a blow molding operation is the ability to provide a blow molded container body having a neck finish of close dimensional tolerance onto which a closure is adapted to be applied. Blow molding threads into a neck finish, for example, is difficult. In fact, it is often necessary to perform subsequent neck finishing operations, after blow molding, in order to obtain or retain the close dimensional requirements of a neck finish, whether threads, crowns, lugs or otherwise.
Despite significant progress in this art and various alternative blow molding methods, there is still a need and a demand for further improvement. Accordingly, a new and improved method is desired which produces a pair of biaxially oriented container bodies in a blow molding process having neck portions which do not require subsequent finishing operations to assure the dimensional tolerance of the neck finish.